The present invention claims priority from Japanese Patent Applications No. 11-315320 filed Nov. 5, 1999, No. 2000-191817 filed Jun. 26, 2000 and No. 2000-311196 filed Oct. 11, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a semiconductor device tester using electron beam and, particularly, to a semiconductor device tester in which current flowing through a semiconductor device irradiated with electron beam is measured.
2. Description of Related Art
In a semiconductor device such as memory, contact-holes or via-holes are usually provided for electrically connecting active elements formed in a lower portion thereof to a wiring layer formed in an upper portion thereof. The contact-holes are formed by etching an insulating film such as an oxide film from a surface thereof down to an underlying substrate by reactive ion etching. In order to optimize etching condition, it is necessary to detect an outer and inner configurations of a contact-hole or a state of a bottom of the contact-hole.
Since the diameter of contact-hole is in the order of microns or less, visible light can not illuminate the bottom of the contact-hole, so that it is difficult, to detect the state of the contact-hole optically. Therefore, SEM (Scanning Electron Microscope) suitable for analysis of a fine structure has been mainly used as a tester. In the SEM, a contact-hole region is irradiated with electron beam, which is accelerated to several tens keV and collimated to several nanometers, and secondary electron produced in the irradiated region is detected by a secondary electron detector, on which an image of the contact hole is formed. A specimen irradiated with the electron beam generates secondary electrons, an amount of which corresponds to constituting atoms thereof However, the secondary electron detector in the SEM is usually arranged in a specific direction, so that a whole of produced secondary electrons are not always detected. If the specimen includes irregularity in its structure, there is a case where secondary electron is not detected depending upon portions of the specimen, resulting in that contrast is produced in an image of the specimen under test, which is form ed of a single substance. This is the feature of the SEM.
On the other hand, an electrical contact is realized through a contact-hole or a through-hole. Therefore, not only a configuration of an opening portion of the contact-hole but also a configuration and a surface condition of the bottom portion of the contact-hole are very important. In an etching for forming contact-holes each having aspect ratio exceeding 10 in concomitant with the recent increase of integration density and the number of layers of a semiconductor device, there may be a case where inner diameters of the contact-holes become different from diameters of opening portions of the contact-holes depending upon process condition even when the sizes of the opening portions are the same as a designed size. Since such variation of the inner size of the contact-hole substantially affects characteristics of a semiconductor device, it is necessary for persons in charge of a process to control the process such that all contact-holes have identical sizes. Further, since such size variation of contact-holes must not exist in practical products, the products have to be tested too. Therefore, a technique capable of non-destructively detecting both the inner size of the contact-holes and such size variation of the contact-holes is very important.
FIGS. 4(a) and 4(b) illustrate a test method using an SEM for testing a contact-hole 43 having a circular cross section and a result of the test thereof, respectively, and FIG""S. 5(a) and 5(b) illustrate a test method using an SEM for testing a tapered contact-hole and a test result thereof, respectively. In the test using the SEM, the specimen under test is scanned by electron beam 31 and secondary electron 32 produced in the specimen is detected by a secondary electron detector 33.
It is assumed that the circular contact-hole 43 is formed through an insulating film 41 such as an oxide film formed on an underlying substrate 42 by etching from an opening portion thereof in a vertical direction such that the contact-hole 43 has an inner diameter substantially equal to a diameter of the opening portion, as shown in FIG. 4(a). In such case, energy of secondary electron hardly reaches the detector 33 unless there is a space large enough to gather a sufficient amount of energy since the energy of secondary electron is small. Therefore, a measured amount of secondary electron becomes as shown in FIG. 4(b). That is, an image of secondary electron obtained becomes suddenly darkened correspondingly to the opening portion of the contact-hole 43. By this phenomenon, an existence of a contact-hole is detected.
On the other hand, it is assumed that a contact-hole 44 has a tapered configuration whose diameter is reduced with depth as shown in FIG. 5(a). In such case, secondary electron from the tapered portion of the contact-hole may be observed depending upon a position of a secondary electron detector. However, since the aspect ratio of the contact-hole 44 is large, secondary electron emitted from an inner wall of the contact-hole can not observed practically. Therefore, the configuration of the contact-hole 44 and an information of a bottom thereof are not always reflected to a secondary electron image.
In the tapered contact-hole such as shown in FIG. 5(a), the inner diameter thereof is reduced with increase of a depth thereof and there may be a case where a contact resistance of the contact-hole is increased, resulting in a defective contact-hole even if the diameter of the opening portion thereof is acceptable. In the SEM test, however, a detected image becomes dark sharply at the opening portion of the contact-hole and an information of a bottom thereof is not reflected to the image regardless of whether the configuration of the contact-hole is circular or tapered Thus, it is impossible to distinguish these contact-holes by the usual SEM.
In order to test an interior or a bottom of a contact-hole, a method of observing a cross section of the contact-hole of a specimen obtained by vertically cutting the specimen along a center axis of the contact-hole has been employed. This method requires a high level technique for precisely cutting the specimen to two pieces at the center axis of the contact-hole. Therefore, in view, of the diameter of the current contact-hole in the order of several thousands xc3x85, it is practically impossible to cut the specimen along the center axis of the contact-hole with precision of 10% which is necessary to determine the quality of a product. Further, this method is a destructive test and requires considerable labor and time, in addition to the impossibility of directly observation of the product.
In order to solve such problems, JP H10-281746A discloses a technique in which current produced by electron beam, which is passed through a contact-hole and arrived at a substrate, is detected to detect a position and size of a bottom of the contact-hole. Further, JP H4-62857A discloses a technique in which a secondary electron image is obtained by irradiating a contact-hole with not electron beam but ion beam and measuring a current flowing through a substrate due to the ion beam irradiation.
As another prior art, JP H11-026343A discloses a technique in which a pattern for measuring a positional deviation of a mask is formed and an amount of positional deviation of the mask is obtained on the basis of a substrate current produced when electron beam irradiation is performed. Further, JP P2000-174077A discloses a technique in which an area containing a plurality of contact-holes is irradiated with electron beam and a ratio of normal contact-holes in that area is tested on the basis of current values produced by electron beam passed through the contact-holes.
Further, it is possible to know a film thickness by measuring a substrate current. For example, JP P62-19707A discloses a technique in which a relation between a waveform of a substrate current, acceleration voltage of electron beam and a film thickness, when a pulsed electron beam irradiation is performed, is preliminarily obtained and a fir thickness is obtained from a current waveform measured by using electron beam accelerated with a certain acceleration voltage. Further, JP P2000-124276A discloses a technique in which a current, which is not a variation of current with time but a current value, produced by irradiating a test sample with electron beam and passed through the test sample to a backside surface thereof is measured. In a technique disclosed in JP 2000-180143A, a current flowing through a thin film to a substrate, is measured and a film thickness is obtained by comparing the measured current with a current value obtained for a standard sample and JP P2000-164715A discloses a standard sample suitable for use in the technique disclosed in JP P2000-180143A.
An object of the present invention is to further improve the technique for detecting a substrate current produced by irradiation of electron beam to thereby provide a semiconductor device tester capable of non-destructively testing a detailed configuration of a contact-hole and an inner state of a semiconductor device.
The semiconductor device tester according to the present invention, which includes electron beam irradiation means for irradiating a semiconductor device as a sample under test with electron beam while scanning the semiconductor device, current measuring means for measuring current produced in the sample by irradiation of electron beam and data processing means for processing measured data from the current measuring means, is featured by that the electron beam irradiation means includes collimator means for collimating electron beam to parallel beam and means for changing acceleration voltage of electron beam and the data processing means includes means for obtaining an information related to a structure of the sample in a depth direction on the basis of a difference in transmittivity of electron beam for the sample when the latter is scanned with different acceleration voltages.
The reason for the use of parallel electron beam in the present invention is that, when a converging electron beam is used, it is necessary to condense the electron beam to a vertical level of a measuring location and, so, it is not suitable in obtaining an information of the sample in a depth direction thereof. When a parallel electron beam is used, focal distance becomes infinite so that focus regulation becomes unnecessary.
The previously described technique disclosed in JP H1-281746A can perform a test for detecting whether or not the contact-hole penetrates the film. However, it can not provide a detailed information of such as configuration of a contact-hole. This is also true for the technique disclosed in JP H4-62857A, which uses ion beam. Although there is a description in JP P2000-124276A of a change of the amount of current or acceleration voltage of electron beam, the purpose of the change of current or acceleration voltage is to reduce noise, not to check a structure of the test sample in its depth direction. The use of parallel beam disclosed in JP P2000-174077A is to irradiate the area including a plurality of contact-holes, not to check the structure such as individual contact-holes of the test sample in the depth direction thereof.
The electron beam irradiation means includes an electron gun and the collimator means includes a condenser lens for collimating the electron beam emitted from the electron gun to a parallel beam and an aperture plate having an aperture inserted between the condenser lens and the semiconductor device, for limiting a spot size of electron beam such that the electron beam impinges an opening portion. The electron beam irradiation means preferably includes means for moving the sample under test with respect to the electron beam in order to scan the a sample with electron beam.
Alternatively, the electron beam irradiation means includes an electron gun and the collimator means may include a first condenser lens for collimating the electron beam emitted from the electron gun to parallel beam, a second condenser lens arranged such that it constitutes an afocal system, an objective lens and an aperture plate having an aperture inserted into between the first condenser lens and the second condenser lens for limiting a spot size of the electron beam. It may further include means for moving the sample under test with respect to the electron beam in order to scan the sample with electron beam.
The electron beam irradiation means may include means for vertically irradiating the semiconductor device along a line segment passing through a center of a measuring region with electron beam having spot size smaller than an area of the measuring region and the data processing means may include means for obtaining a distance of a bottom of the measuring region from a time between a rising and falling edges of a current measured along the line segment.
The data processing means may include area calculation means, which divides a value of current produced in an unknown measuring region by electron beam irradiation under constant condition by a value of current produced in a standard sample having a known area of measuring region by electron beam irradiation under the same constant condition and obtains an area of the unknown measuring region from a resulting quotient. In this case, the data processing means may include distance calculation means, which divides the area obtained by the area calculation means by the ratio (xcfx80) of the circumference of a circle to its diameter and obtains a root of the resultant quotient as a distance measured from one edge to the other of the unknown measuring region.
The electron beam irradiation means may include means for setting the spot size of electron beam to a value large enough to cover all of the measuring region in the lump and the data processing means may include means for calculating a ratio of current produced when the standard sample having the known measuring region area is irradiated with electron beam having the large spot size to a value of current produced when a measuring region of the unknown sample is irradiated with electron beam having the large spot size and means for calculating an area of the measuring region of the unknown sample from the ratio.
The data processing means may include means for determining the value of current produced when the standard sample is irradiated with electron beam having known spot size as an amount of current per unit area of the standard sample.
The data processing means may further include means for comparing a current measured correspondingly to a positional coordinates if a wafer under test irradiated with electron beam with a current to be measured at the same positional coordinates of the wafer is good and setting the kind of process to be performed next on the basis of the result of the comparison.
The present invention can be utilized in combination with an SEM. That is, the semiconductor device tester according to the present invention further comprises a secondary electron detector for detecting secondary electron emitted from a surface of a sample under test, wherein the data processing means may include correspondence means for making an amount of secondary electron measured by the secondary electron detector correspondent with the result of measurement of the current measuring means. In detail, it is possible to vertically irradiate the sample under test along the line segment passing through a center of a measuring region with electron beam having spot size smaller than an area of the measuring region by means of the electron beam irradiating means, obtaining a bottom distance of the measuring region from a distance between a rising and falling edges of current measured along the line segment by means of the current measuring means and obtaining an upper distance of the measuring region from a distance between a rising and falling edges of the secondary electron detected by the secondary electron detector. The correspondence means may include means for three-dimensionally displaying a circular pillar or a frustum of a cone having the measured bottom distance, upper distance and film thickness of the measuring region as a bottom diameter, an upper diameter and a height.
The semiconductor device tester further comprises tilting means for tilting a sample stage on which a sample under test is mounted, wherein the data processing means may include means for processing a tilting angle of the sample with respect to electron beam.
The data processing means may includes means for storing a current value corresponding to an electron beam irradiated portion, which is obtained in a location of the sample having no dust (particle or residue), means for comparing the current value stored in the storing means with a current value corresponding to an electron beam irradiated position in the same pattern portion of an unknown sample as that of the sample and means for determining the existence and size of dust from a difference between a rising and falling positions of the current obtained by the comparison.
The electron beam irradiation means may include means for setting a cross sectional shape of electron beam such that it covers the whole measuring region in the lump and at least one end of the cross sectional shape of electron beam becomes linear and the data processing means may include means for obtaining the distance of the measuring region from a distance between a rising value and a maximum value of current.
The electron beam irradiation means includes means for setting a cross sectional shape of electron beam such that it covers a whole measuring region in the lump and at least one end of the cross sectional shape of electron beam becomes linear and the data processing means may include means for calculating a differentiated curve of current with respect to a distance and means for obtaining a radius of the measuring region from a distance between a rising position and an apex position of the differentiated curve.
The data processing means may include means for displaying measured current values on a map corresponding to the measured positions.
The data processing means may include comparison means for comparing a measured value obtained in one of two regions on a wafer as samples with a measured value obtained in the other region as a reference value and means for extracting a positional coordinates when there is a difference equal to or larger than a predetermined constant value.
In this case, the electron beam irradiation means includes means for scanning a sample under test with line shaped electron beam having length substantially equal to a width of a wiring in a direction perpendicular to a lengthwise direction of the line shaped line and moving a scan position by a distance equal to the width of the wiring vertically to scanning direction after one line scan is completed and the comparison means may include means for comparing current waveforms measured as variations of current values with respect to electron beam irradiating positions in the two regions.
The electron beam irradiation means includes means for scanning a sample under test with electron beam having size smaller than a minimum width of a wiring of the sample in a first direction and moving the scan position in a direction perpendicular to the scanning direction by a distance corresponding to the width of the wiring every time one line scan is completed and the comparison means may include means for extracting, from current waveforms measured as variations of current values corresponding to electron beam irradiating positions in the two regions, instantaneous current values at centers of a rising and falling edges of the current waveform corresponding to the same pattern positions and comparing the instantaneous current values with each other.
The electron beam irradiation means includes means for scanning a sample under test with line shaped electron beam having a length capable of irradiating a plurality of wiring lines of the sample as a lump in a direction perpendicular to a lengthwise direction of the line shaped electron beam and moving the sample in a direction perpendicular to the scanning direction by a width of electron beam irradiating a scan position every time when one line scan is completed and the comparison means may include means for comparing current waveforms measured as variations of current values for electron beam irradiating positions in the two regions. In this case, the means for comparing waveforms may include means for integrating the waveforms and comparing the integrated values.
The comparison means may include means for integrating current from a rising edge to a falling edge of one pulse of a current waveform measured as a variation of a current from an electron beam irradiating position, divider means for dividing the integrated value by a distance between the rising edge and the falling edge of the pulse and means for comparing current values per unit area of the two regions obtained by the divider means.
The comparison means may include means for comparing positions of a rising edge and a falling edge of the pulse of the current waveform measured as a variation of current value for an electron beam irradiating position. Alternatively, the comparison means may include means for comparing the center position of the rising edge and the falling edge of that pulse.
The electron beam irradiation means may include main scan means for moving a sample under test with respect to electron beam and sub scan means for repeatedly deflecting electron bean in a direction different from a main scan direction simultaneously with the main scan.
The electron beam irradiation means can switch an operation mode between a first mode in which individual wiring lines of a sample under test are irradiated with electron beam and a second mode in which all of the wiring lines of the sample are irradiated with electron beam in the lump and the data processing means may include means for analyzing, every constant positional section, spacial frequency of current waveform measured as a variation of current value for electron beam irradiating position in the first mode and detecting a position in which sections having the same spacial frequency continue for a constant time period and means for, under an assumption that a plurality of wiring lines are arranged in an array in the detected position, setting the electron beam irradiation means to the second mode and obtaining defect ratio in the lump.
The means for obtaining information related to the structure in the depth direction preferably includes means for obtaining a three-dimensional configuration of a through-hole provided in an insulating film by measuring values of current produced by irradiation of electron beam passing through a portion of the insulating film, which surrounds the through-hole, with increased acceleration voltage.
In order to obtain a three-dimensional configuration of a through-hole provided in an insulating film, it is necessary to know a thickness of the insulating film. The technique disclosed in JP S62-19707A, P2000-124276A or P2000-180143A may be used therefor.
The semiconductor device tester may further include means for tilting a sample stage on which a sample under test is mounted and the means for obtaining the three-dimensional configuration preferably includes means for detecting whether a diameter of a through-hole is increased or decreased with depth, from measured values obtained when electron beam and an incident angle of electron beam to the sample are changed.
The means for obtaining the information related to a structure in a depth direction may include means for detecting deviation of a circuit pattern in an insulating f from measured value of current produced by electron beam passing through the insulating film.
Although a technique for measuring a deviation of mask position is disclosed in JP H11-026343A, the measurement of the mask position deviation utilizes a measuring pattern with which a through-hole is provided when the mask positions are registered. It does not use electron beam passing through an insulating film.
The means for detecting deviation of circuit pattern preferably includes means for evaluating a deviation of circuit patterns in respective layers from measured values when penetrating depth of electron beam is changed by changing acceleration voltage. In order to obtain a position of the insulating layer in which the circuit patterns overlap, means for taking in an information related to the circuit patterns from CAD data is preferably provided.
In the construction mentioned above, acquisition of current waveform is performed by electron beam scanning and measured current contains current flowing through a capacitance of a sample depending upon irradiation frequency or scanning frequency. Therefore, there may be a case where D.C. current, which can not flow through the sample essentially, is measured as if it flows through the sample. In order to avoid such phenomenon, the data processing means preferably includes means for correcting current component flowing through a capacitance of a sample under test, which is caused by irradiation frequency of electron beam or scanning frequency. In detail, in a case where the electron beam irradiation means has a construction in which pulsed electron beam is generated repeatedly, it includes means for changing the repetition period of electron beam pulse and the correcting means preferably includes means for obtaining the D.C. component by extrapolating current value when the sample is continuously irradiated with electron beam from current values measured by the current measuring means when the sample is irradiated with electron beam with different repetition period. The semiconductor device tester may further include means for switching scan speed of electron beam from the electron beam irradiation means and the correcting means may include means for obtaining a current value when the scanning speed, which is zero, is extrapolated from the current values measured by the current measuring means when the sample is scanned by electron beam at different scan speeds.